The 7 AHRQ funded Centers for Education and Research on Therapeutics (CERTs), led by the HMO Research Network CERTs (10 HMOs), will collaborate to improve the detection of medication prescribing errors in the ambulatory setting, and to develop new uses of this information to improve care. These HMOs, which include staff/group, network, and IPA systems, have nearly 16,000 primary care providers who care for approximately 7,000,000 people in over 1,000 locations. To detect errors we will use existing automated pharmacy dispensing information and automated inpatient and outpatient diagnosis and procedure data, plus limited full text record review, to assess the overall frequency of prescribing at variance with either FDA "black box" warnings or clinical guidelines. We will also focus on a) errors in high risk populations, b) incorrect co-prescribing of interacting drugs, c) failure to adjust dosage for renal insufficiency, and d) failure to perform laboratory monitoring for toxicity. To prevent the errors with the greatest impact, we will develop and implement interventions that operate through existing institutional quality assurance/quality improvement bodies, through targeted reminders to patients, or through delivery of information to prescribers via order-entry systems. Several of these interventions will also use academic detailing. The HMOs' data systems will be used to assess the impact of these interventions on error rates. We will also examine various methods of informing patients about medication errors that have resulted in harm, using telephone interviews based on scenarios developed from actual errors. This work will provide a much-needed assessment of the frequency of prescribing with error in the ambulatory setting, and it will develop methods to use this information to improve patient safety. Some of these interventions will be immediately applicable to organizations that care for the large majority of the US population. Others will be ready for use as ambulatory order entry prescribing systems are disseminated.